Prior propylene-ethylene copolymers have been extensively used in the field of films or the like, by utilizing the characteristics of lower crystallizability and lower glass transition point than linear crystalline polypropylene homopolymers. However, the use has been limited in other application fields, and more improved characteristics have been required for use in those applications.
For instance, low temperature heat-sealing characteristics have been required from a standpoint of energy-saving even in the film field as most extensively used. The prior art has employed a method of lowering the melting points of the copolymers with the sacrifice of the film stiffness. Thus it has been strongly demanded to reconcile stiffness and low temperature heat-sealing characteristics which conflict with each other.
These prior propylene-ethylene copolymers are usually produced by copolymerizing ethylene and propylene using a titanium catalyst. However, at improvement in various properties of the propylene-ethylene copolymers produced by such copolymerization method is considered to reach substantially the limit. In recent years, there have been investigated various methods for producing olefin (co)polymers by (co)polymerizing olefins using different catalyst systems in which metallocenes are combined with aluminoxanes.
For instance, Japanese Patent Kokai 3-12406, Japanese Patent Kokai 3-12407 and CHEMISTRY LETTERS, pp. 1853-1856, 1989 disclose that high stereoregular polypropylenes produced by polymerizing propylene using catalysts consisting of silylene-bridged metallocenes having the specific structures and aluminoxanes have narrow molecular weight distributions, high melting points and high rigidities. However, no concrete technique is disclosed therein on propylene-ethylene copolymers.
Further, Tsutsui et al. have considered that, for the propylene-ethylene copolymers produced by copolymerizing propylene and ethylene using an ethylenebis(1-indenyl)zirconium dichloride and methylaluminoxane catalyst system, the stereoregularities thereof defined by the meso--meso triad sequence are the same as those of the copolymers produced with prior titanium-containing catalyst components, but the melting points of said propylene-ethylene copolymers produced with said metallocene and aluminoxane catalyst system are lower than those of the copolymers obtained with the titanium catalyst components, and the causes are due to larger amounts of inverted propylene units in the propylene-ethylene copolymers obtained with the metallocene-aluminoxane catalysts (T. Tsutsui et al., POLYMER, 1989, Vol. 30, 1350). Larger amounts of inverted propylene units are due to the fact that the polymerization of propylene using the titanium catalysts proceeds substantially with 1,2-insertion, whereas the polymerization of propylene using known metallocene catalysts proceeds with 2,1- and 1,3-insertions in a constant ratio.